The Portland Observatory is a historic maritime signal tower at 138 Congress Street in the Munjoy Hill section of Portland, Maine. Built in 1807, it is the only known surviving tower of its type in the United States. Using both a telescope and signal flags, two-way communication between ship and shore was possible several hours before an incoming vessel reached the docks. The tower was designated a National Historic Landmark in 2006; it is now managed by a local nonprofit as a museum, and is open to the public. The Portland Observatory is the only remaining maritime signal station in the United States. Tower operations were paid with annual fees collected from shipping merchants, who purchased the right to have their flags stored in the building and hoisted up its flagstaffs when their ships were sighted. A telephone was eventually installed, extending the tower's function until 1923, when the reliability of engine powered vessels and communication by radio made it obsolete.